Diagnosis and Management of Large Intestinal Disorders in Dogs

Todd R. Tams, DVM, Dipl. ACVIM
VCA West Los Angeles Animal Hospital



Large bowel disorders are common in dogs. These disorders can usually be managed very successfully. It is useful in any patient with diarrhea to begin by attempting to differentiate between primarily small bowel and large bowel diarrhea, based on presenting signs and characteristics of the stool. Tests and treatment often vary for small and large intestinal disorders, making this initial characterization very important. Because large bowel - type problems occur so commonly, I often begin by asking questions relative to this area of the intestinal tract when presented with a patient with diarrhea. Specifically, the presence or absence of mucus, fresh blood, straining, and any change in frequency of defecation are discussed.

Small bowel diarrhea is often characterized by an increased frequency of defecation with evacuation of larger than normal amounts of soft stool. Dyschezia and tenesmus are not characteristics of a small bowel disorder and are apparent only if a large bowel disorder is present as well (this is an important historical point, indicating probable diffuse intestinal involvement). Urgency may be present in acute small bowel disorders or in those associated with cramping. Generally, rapid evacuation of a large volume of watery diarrhea ensues (as opposed to large bowel problems in which only a small volume is passed). Presence of undigested food indicates maldigestion, which is generally due to either EPI or rapid bowel transit time.

The presence of weight loss and inappetence in conjunction with chronic diarrhea suggests a significant small intestinal disorder (e.g., inflammatory bowel disease, lymphangiectasia, histoplasmosis, neoplasia), and their presence should hasten the clinician's efforts toward making a definitive diagnosis. The combination of chronic diarrhea, weight loss, and increased appetite in cats suggests hyperthyroidism, inflammatory bowel disease, EPI (rare in cats), and occasionally lymphosarcoma (some cats with GI lymphoma actually have an increased rather than decreased appetite). This combination of signs in dogs is most consistent with EPI. Characteristics of diarrhea in animals with EPI include voluminous soft consistency stools that are often rancid in nature. Coprophagy is an ancillary sign that frequently occurs in dogs with EPI. Weight loss and inappetence rarely occur in dogs and cats with intestinal disorders limited to the large bowel.

Physical Examination

Along with the history, physical findings help direct the clinician regarding what specific tests, if any, should be done and how quickly a work-up should be expedited. Particular attention is paid to the animal's attitude, hydration, and posture. Abnormal posture (e.g., arched back) may indicate abdominal pain that can be associated with acute or chronic disorders. Body weight and overall physical stature should be noted. The act of defecation, especially if there is a history of dyschezia or tenesmus, should be observed by the clinician whenever possible.

Careful abdominal palpation is done to examine for thickened bowel (inflammatory or neoplastic infiltration), intussusception, presence of a mass that could be causing partial intestinal obstruction with resultant diarrhea, and lymphadenopathy (benign or neoplastic). The caudal dorsal abdominal region should be palpated in dogs with signs of large bowel diarrhea in order to see if there is evidence of discomfort. A rectal examination is always done in dogs with diarrhea, of any type, to examine for increased mucosal sensitivity, presence of narrowing (e.g., infiltrative disease, stricture), foreign body, or mass effect and to obtain a fresh stool sample for gross examination.


In mild cases a diagnosis is often established based on fecal parasite examination (e.g., whipworms, hookworms, coccidia, and Giardia); positive response to empirical treatment for difficult-to-diagnose parasite problems (Giardia and whipworms); response to dietary trials (fiber augmented diet, elimination diets); or response to empirical treatment for acute colitis. Diagnostic tests for chronic large bowel diarrhea principally involve:

1.  Fecal cytology to look for increased numbers of C. perfringens spores and inflammatory cells (specifically neutrophils), which suggest bacterial or primary inflammatory disease.

2.  Fecal culture if history or fecal cytology suggests the possibility that bacterial infectious disease exists (Campylobacter, Salmonella). Fecal cultures are not commonly indicated in small animal patients.

3.  Reverse passive latex agglutination enterotoxin assay on stool to confirm a diagnosis of C. perfringens enterotoxicosis (test of choice for CPE).

4.  Colon biopsy via colonoscopy (preferred technique) or surgery (see later discussion).


Complete colonoscopy with examination of the rectum, descending, transverse, and ascending colon, cecum, and ileocolic orifice area is preferred. Although examination and biopsy of the descending colon with a rigid colonoscope is commonly diagnostic in animals with large bowel diarrhea, such problems as occult trichuriasis in which whipworms may be grossly evident in the cecum but not in the descending colon, ileocolic or cecocolic intussusception, typhlitis, or neoplasia that is localized in the transverse or ascending colon may be missed unless a complete examination of the colon is done with a flexible endoscope. Another advantage of using a flexible endoscope is that ileoscopy may be accomplished in after complete colonoscopy. Biopsy samples should always be obtained during colonoscopy, regardless of gross appearance.

The primary indications for performing colonoscopy are for chronic recurrent large intestinal type diarrhea, suspected chronic small intestinal disease in patients in which both upper and lower small intestinal biopsies are desired (both duodenum and ileum), and for evaluation of dyschezia, hematochezia without abnormal stool consistency, and for evaluation of a mass or possible intussusception (cecocolic or ileocolic).

Clostridium perfringens Enterotoxicosis

See notes on this important cause of large bowel diarrhea elsewhere in this proceedings.

Management of Large Intestinal Diarrhea

Treatment of large intestinal diarrhea frequently involves dietary manipulation, specific anthelmintic therapy for parasite infections, antibacterial drugs or antifungal agents for infectious disorders, and antiinflammatory therapy for large intestinal inflammatory bowel disease (sulfasalazine and metronidazole are the most common drugs used to control inflammation of the large intestine in dogs). Symptomatic therapy for acute non-complicated diarrhea includes bowel rest and dietary manipulation. The treatment for C. perfringens enterotoxicosis has already been described above. If pharmacotherapy is deemed necessary for acute large intestinal diarrhea, metronidazole is often the most indicated drug and is frequently used for 5 to 7 days on an empirical basis. It is emphasized that some animals with chronic diarrhea may have several disorders at the same time (e.g., inflammatory disease, C. perfringens enterotoxicosis, and small intestinal bacterial overgrowth). A thorough work-up will lead to diagnosis of each disorder, with subsequent development of a comprehensive treatment plan. The likelihood of more rapid resolution of symptoms is much greater when each existing problem is properly treated.

Sulfasalazine (Azulfidine) is a drug that is commonly used for colitis in dogs. It is used somewhat less commonly in cats, primarily because corticosteroids are very effective in controlling colitis in cats, whereas corticosteroids are rarely effective in dogs with colitis (unless the disorder is primarily eosinophilic colitis, which is very uncommon in my experience). Sulfasalazine is a combination of 5-aminosalicylic acid (ASA) and sulfapyridine tied together by an azo bond that prevents significant absorption of the drug before it reaches the colon. Once in the colon, where the bacterial count is considerably higher than the small intestine, bacteria split the bond and release the 5-ASA for its local effect. Olsalazine (Dipentum), Asacol, and Pentasa are drugs that contain only 5-ASA combined by an AZO bond. These drugs can reach the colon in higher concentration that Azulfidine. They are more expensive, however, and dosage preparations are limited. I still use Azulfidine in most dogs in which use of a 5-ASA containing drug is desired.

The starting dose of Azulfidine in dogs is generally 10-15 mg/lb TID. In more chronic or severe cases a dose of 15-25 mg/lb TID is recommended. The dose should not exceed a total of 5 grams per day. Side effects are uncommon, but may include keratoconjunctivitis sicca (KCS), allergic dermatitis, nausea and vomiting, and cholestatic jaundice. In my experience, KCS occurs quite uncommonly in dogs on sulfasalazine. I do, however, routinely take the precaution of doing a Schirmer tear test on middle age to older dogs before instituting sulfasalazine. Duration of therapy is quite variable. In mild colitis cases, 7-14 days of therapy with sulfasalazine may be sufficient, while in others several months may be required. In some dogs with chronic unrelenting colitis sulfasalazine may be needed for months to years. In these cases the lowest possible frequency is used. For example, the dose may be gradually reduced to a BID and then SID schedule, and in some cases a single dose given every other day may be effective on a long-term basis.

Sulfasalazine works well in combination with metronidazole. If clinical signs suggest significant patient discomfort and/or biopsies reveal moderate to severe large intestinal disease I will frequently administer both drugs in combination.

Metronidazole has both an antibacterial and antiinflammatory effect. It is useful in treatment of both small and large intestinal inflammation. Metronidazole's mechanism of action includes an antiprotozoal effect, inhibition of cell-mediated responses, and anaerobic antibacterial activity. Metronidazole is administered at 5 to 10 mg/lb two times daily. Also, I have successfully managed some canine patients with mild to moderate lymphocytic-plasmacytic colitis on a long-term basis with metronidazole.

Irritable Bowel Syndrome

The irritable bowel syndrome (IBS) is a chronic disorder characterized by variable signs of gastrointestinal dysfunction or distress in the absence of underlying structural causes for the symptoms. It has also been termed spastic colon, nervous colon, spastic colitis, and mucous colitis. IBS has been well recognized in man for many years and is classified as a functional bowel disorder. The term functional disorder indicates disordered or abnormal physiologic function in the absence of pathologic lesions. IBS in humans is characterized by intermittent disturbances of intestinal function with unpredictable periods of exacerbation and remission. Sometimes bouts occur in response to nonspecific emotional stresses or other psychologic factors. Clinical experience, based on detailed patient evaluation (including negative enteroscopies and colonoscopies) and no evidence of organic disease in a number of patients with compatible signs, strongly suggests that a similar syndrome occurs in small animals, especially dogs.

The clinical presentation in dogs with IBS most often involves signs of a large bowel disorder. Intermittent passage of small amounts of mucoid stool, with or without dyschezia, and increased frequency of defecation are commonly observed. Hematochezia may occur but is infrequent. There are occasional episodes of urgency to defecate, perhaps because of a sensation of incomplete evacuation. Stools may be soft but formed or watery. Unlike the situation in humans, in my experience, constipation does not commonly occur in dogs with IBS.

Often, other signs occur, with or without diarrhea, that heighten the clinician's suspicions that IBS may be present. Intermittent bloating, nausea, vomiting, and abdominal pain may also be present. There may be evidence of significant distress (e.g., reluctance to move, groaning, pacing) associated with cramping. It is emphasized that IBS is a chronic disorder characterized by an intermittent or cyclic pattern of symptoms. The clinician should evaluate the history carefully, with particular interest in determining what, if any, stressful or disruptive events occur in close association with clinical signs. Stressful factors may include being left alone for a longer period of time than the animal is accustomed to, household conflict between humans, or other environmental stimuli, such as work details for police or other performance dogs. Pecking order problems with other dogs in the immediate environment may also be a factor. While some dogs with IBS are timid and reserved or, alternatively, hyperactive, many other dogs seem to be well adjusted, and stressful factors cannot be reliably implicated as a cause. In these situations, distinct periodic aberrations in normal motility patterns (involving either small or large bowel or both) may be involved, and stress may play no role at all. Alternatively, perhaps over-responsive, "doting" owners may themselves represent a significant "stress" factor.

There is no known sex or breed predilection for IBS. I have examined a number of both large breed dogs (police, guard, seeing eye) and small to medium-size dogs in which clinical signs were consistent with IBS and in which detailed diagnostic evaluations failed to provide any positive diagnostic information for the presence of organic disease. It is strongly suggested that a diagnosis of IBS be limited to patients that have undergone detailed diagnostic evaluation, lest the term be used too loosely to describe conditions with a provable cause for which the clinician has failed to investigate thoroughly.


The diagnosis of IBS is complicated by the fact that similar clinical signs can be caused by a number of gastrointestinal disorders (i.e., functional disorders may resemble the presentation of organic disorders) potentially involving any area of the gastrointestinal tract. Since there are no histologic changes identifiable in patients with IBS, no hematologic or biochemical abnormalities, and no pathogens to identify on culture or serology, IBS is truly a diagnosis of exclusion. The clinician can only be confident of a diagnosis of IBS once all other conditions have been excluded. In my experience, abnormal gastric motility, inflammatory bowel disease, and chronic idiopathic colitis are the most commonly encountered disorders that must be differentiated from IBS. As the treatment course and prognosis for these disorders can vary considerably, it is strongly recommended that patients with chronic signs be thoroughly evaluated so that the best possible therapeutic regimen can be instituted.

If clinical signs are poorly responsive to initial treatments, the clinician should undertake a more detailed investigation, including a complete blood count, biochemical profile, and fecal cytology. (Increased numbers of neutrophilic leukocytes suggest inflammatory small or large bowel disease or invasive bacterial enteritis.) Since chronic C. perfringens enterotoxicosis can cause chronic signs very similar to those of IBS, fecal toxin analysis should be done to evaluate for this problem. Contrast radiographic studies (food mixed with barium or radiopaque markers - BIPS) should be considered if a gastric motility disorder is suspected (stomach should be empty by 8 to 10 hours after a meal). Contrast studies are generally done if the patient is primarily experiencing signs referable to a gastric or gastroesophageal problem. If small and large intestinal signs predominate, I generally prefer to do gastroscopy, enteroscopy (both duodenum and ileum), and colonoscopy as the next step after blood and fecal analyses are completed. The diagnostic yield is considerably greater with endoscopy than with survey and contrast radiography. Since inflammatory disease of the small and large intestine more commonly cause symptomatology similar to that of IBS than any other GI disorder, histologic examination of the intestine is extremely important. Intestinal biopsies are normal in IBS patients.


Management of IBS poses a significant challenge to both primary care veterinarians and specialists. Because symptoms and timing of episodes are so variable, response to treatment is sometimes unpredictable, and the disorder is incurable, clients can easily become frustrated with their inability to effect better control over their pet's condition. One of the most important initial steps for the veterinarian to undertake is to secure the client's confidence. Credibility is best established by thoroughly excluding organic causes and then spending time educating the client about the nature of IBS. Reassurance that no serious disease exists and that life expectancy should not be altered with IBS should be conveyed. Next, a therapeutic strategy for control of symptoms is formulated and tailored to the predominant clinical signs of the patient. General categories of primary symptomatology include diarrhea (most common), abdominal pain, and vomiting, or a combination of these signs. Treatment generally includes, either individually or in combination, dietary modification and use of fiber supplementation, antidiarrheal drugs, anticholinergics, and tranquilizers. Most patients can be managed successfully with dietary adjustments and intermittent pharmacotherapy.

Potentially beneficial effects of fiber include alteration of abnormal intestinal myoelectrical activity and normalization of gut transit times. In human studies that support fiber efficacy, the symptoms that most often improve include constipation and abdominal pain. Fiber supplementation has been used in canine IBS patients with variable results. In my experience, only a small number of IBS patients with chronic diarrhea experience complete resolution of clinical signs on a long-term basis when fiber supplementation is the sole therapy used. Patients whose symptom pattern includes acute, intermittent (days to weeks) flare-ups of large bowel diarrhea sometimes have fewer episodes, but clinical signs rarely abate completely. Still, it makes sense to initiate dietary trials for IBS because even a partial response may be quite beneficial.

Pharmacotherapy for diarrhea-predominant IBS includes use of motility-modifying drugs such as loperamide (Imodium) at 0.2-0.5 mg/lb, PO, or diphenoxylate (Lomotil) at 0.1-0.22 mg/lb, PO, BID. Loperamide is a potent antidiarrheal drug that decreases intestinal secretions, enhances absorption, stimulates rhythmic segmentation contractions, and increases anal sphincter tone. There is often significant improvement in stool consistency and abatement of pain and urgency following loperamide therapy Although loperamide can be used safely on a long-term basis, several days to 1 to 2 weeks of therapy is often sufficient to normalize stools. After the first several days of therapy, it may be possible to decrease administration to once or twice daily.

Patients with abdominal pain (cramping, bloating, assuming an arched-back stance, reluctance to move, loud abdominal gurgling sounds) or those with signs of general distress, such as pacing, are treated with combination antispasmodic-tranquilizer preparations. Chlordiazepoxide (a centrally acting sedative) and clidinium bromide (an anticholinergic agent) are combined in the capsule preparation Librax (Roche). Chlordiazepoxide is a benzodiazepine with peripheral smooth-muscle relaxant properties as well as central nervous system (CNS) effects. This combination seems to be especially effective in relieving the discomfort that may be associated with increased colonic motor function. The dose of Librax (chlordiazepoxiode 5 mg and clidinium 2.5 mg) is 0.2-0.5 mg/lb of clidinium, PO, BID-TID. The drug is generally used on a short-term basis (1 day to 2 weeks), and clients are instructed to administer it at the first sign of cramping or abdominal pain. Occasionally, long-term use is necessary (one to two doses a day). As some IBS patients are affected by unpredictable flare-ups of abdominal distress, a supply of this drug should be kept at home for immediate use.

Combination therapy may be necessary in some cases. For example, a patient with signs predominantly characterized by diarrhea and abdominal pain may respond better to loperamide and clidinium/chlordiazepoxide used concurrently. Despite any objective reason for its use, sulfasalazine sometimes provides symptomatic relief, especially when used in combination with loperamide or clidinium, for IBS patients with significant dyschezia and increased evacuation of small volumes of loose, mucoid stool. This relief has been observed in patients in which multiple colon biopsies and careful evaluation for pathogenic intestinal organisms have proved negative. Likewise H2 receptor blockers such as famotidine at dosages of 0.25-0.5 mg/lb every 24 hr., PO, used in combination with clidinium or isopropamide, may provide better control of IBS-related nausea or vomiting than either drug alone.

Finally, the patient's environment should be evaluated in an effort to identify stress (i.e., antagonistic) factors, if any exist. These should be altered or alleviated whenever possible. Veterinary clinicians are reminded that being available for communication and providing support for clients whose pets are affected by the variable and unpredictable symptoms of IBS are extremely important in the overall management scheme.